The trend toward reduced emissions, as well as increased demand for improvement in fuel economy, has resulted in significantly increased fuel injection pressures. This trend has also led to the use of electronic controls, which provide precise fuel metering and injection timing. Accordingly, conventional fuel delivery systems for high fuel pressure applications, such as diesel fuel systems, typically include a high pressure fuel injection pump or system, which controls fuel injection rate, quantity and timing, to deliver fuel to an engine, via fuel injectors. These fuel delivery systems may also include a primary low pressure pump, which draws fuel from a fuel tank and delivers fuel to the fuel injection system, via a fuel filter.
The fuel injection pump used in these high pressure applications typically includes a means for venting fuel at the end of injection, or a means for venting fuel from a high pressure accumulator. The work done in pressurizing the fuel is not recovered in the venting process, resulting in an increase in fuel temperature. Also, the supply fuel is used for cooling the electronic components, which further increases the fuel temperature. The engine will also contribute toward heating the fuel.
Heated fuel is returned to the fuel filter or fuel tank via a fuel return line. If the heat in the returned fuel is excessive, a fuel cooler may also be required to prevent heat build up in the fuel tank, therefore limiting the fuel injection pump supply temperature. Excessive fuel injection pump supply temperature would not adequately cool the electronics and the less viscous fuel would not adequately lubricate the close clearance parts of the injection system. Because self regulating diaphragm pumps may not provide adequate fuel flow for total system cooling, the primary fuel pump used in these prior art systems is usually electrically driven and requires external pressure and flow regulation via a pressure regulator.
Further, it is well known that when diesel fuel is at low temperatures approaching the cloud point, paraffin wax crystals will form. To prevent wax choking of the filter after a cold start, electric fuel filter inlet heaters are used in some prior art fuel delivery systems. Because newer diesel injection systems also have higher fuel supply and return flow for system cooling, the size of the electric heater becomes too large to have an adequate de-waxing effect on the increased supply fuel flow. Therefore, it may be desirable to divert warm fuel from the injection system to the unfiltered side of the filter to de-wax the filter after a cold start.
With respect to controlling fuel pressure, the inventor of the present invention has found certain disadvantages with these prior art fuel delivery systems. For example, pressure regulation is performed either completely on the filtered side of the fuel filter (where the pressure regulator senses fuel pressure on the filtered side of the fuel filter and bypasses excess filtered fuel), or completely on the unfiltered side of the fuel filter (where the pressure regulator senses the fuel pressure on the unfiltered side of the fuel filter and bypasses excess unfiltered fuel). Filtered side regulation causes excess flow through the filter element, which increases the pressure drop, reduces the service life and adversely reduces the wax build-up time during cold starts. Unfiltered side pressure regulation causes the fuel pressure on the filtered side of the filter to vary, depending upon the fuel flow through the filter, as dictated by engine demand as well as manufacturing variation of the filter, and service life of the filter.
With respect to fuel temperature control, the inventor of the present invention has found that prior art direct acting temperature controlled diverter systems, which are used to selectively divert warm fuel to the fuel filter to de-wax the fuel filter during cold start, or to the fuel cooler or fuel tank after a cold start, generally do not accommodate the high injection pump return fuel flow of modern fuel injection systems. Further, in certain situations where the primary pump cannot meet the return flow requirements of the fuel injection system, it may be desirable to prevent loss in fuel pressure, by overriding the temperature controlled diverter system, such that some warm fuel is returned to the injection system, rather than the fuel cooler or fuel tank, thereby prioritizing fuel pressure maintenance.
The inventor of the present invention has found that devices that protect the fuel filter by diverting return fuel from the unfiltered side of the fuel filter to the filtered side of the filter when a predetermined pressure differential is exceeded, do not currently appear to exist. Fuel returned to the unfiltered side of the filter is common in newer diesel fuel delivery systems, because of the previously mentioned de-waxing advantage, but also because the returned fuel can sometimes contain wear debris from inside the fuel injection pump. However, under certain conditions to avoid failure of another component in the system, it maybe desirable temporarily divert return fuel to the filtered side of the filter. This could be used for instance, to control high filter differential pressure, that may burst the filter. This feature could also be used for a fuel system that occasionally has return flow considerably exceeding the primary pump output, as well as the flow capability of the filter.